Troy is what most people are thinking.
The only solid clue we have is that whomever stole (or picked up) Jamie's phone (who was the one taking the selfies) is the source of the Arieth pictures
BUT it shouldn't have Chad's info on it, so the blackmailer must have access to both somehow.
Two possible candidates out of left-field that no-one (I think) has mentioned.
Derek: He was in the last photo (and ergo where Jamie may have dropped the phone). He certainly hates the Alphas.
Anthony: He wants to be the Pres maybe and hitting both Chad
and Dawe moves him closer? He may know about Chad and remember, he was the only one to point out that the Arieth photos were selfies and the different hairstyle meant a different time - exonerating the MC. Secretly smart? or he's seen the photos before?